It is often necessary to mix two different gases together.
In particular, the use of fuel gases such as natural gas or liquefied petroleum gas (LPG) requires a process of mixing with air. Mixing is generally performed at pressures close to atmospheric pressure.
In such applications, the fuel gas, which is often available at medium pressure, is expanded to a few millibars in order to be put into contact with the oxidizing gas which is generally atmospheric air.
A defect of the numerous mixer devices that are presently in use lies in the narrow operating ranges over which they can maintain the best possible operating conditions. Thus, when feeding burners or carburetors, it is rare for combustion to be stoichiometric over the entire range of use of the installation. Similarly, with generators that generate a mixture of air and LPG, the calorific value of the gas delivered suffers from the inaccuracy of mixer devices which at present provide a dynamic range that rarely exceeds 10:1.
Existing mixer devices include the following:
discrete-action systems in which the air/gas mixing is preadjusted by orifices of through section and feed pressure that are predefined in such a manner as to obtain the looked-for combustion in a burner or a constant higher calorific value (HCV) in a generator of a mixture of air and LPG. Such devices are in widespread use, in particular for the burners of high and medium power boilers which have systems that are preset for one, two, or three operating rates; PA1 modulating systems in which the sir/gas mixture is kept constant by the combined action of two valves that open synchronously, one for gas and the other for air. Such systems are likewise preadjusted, by means of mechanical cam devices, which make it possible to set the desired mixture for each position of the valve. The same principle is to be found in generators for generating a mixture of air and LPG; and PA1 modulating systems controlled by controllers and computers in which action is taken on gas and air admission valves on the basis of one or more measurements (flue gas analysis, temperature of a process, gas flow rate, etc.). PA1 small dynamic range for flow rates over which the HCV is stable, since dynamic range depends in most cases on the dynamic range of the meter which rarely exceeds 20:1; PA1 it is difficult to stabilize pressure regulation, particularly at low pressure, since the control system needs to govern continuously the precise positions of two valves (one for air and the other for LPG); PA1 they are unsuitable for regimes in which variations are fast because of the relatively long response times due to the two valves under control; PA1 installations consume large quantities of energy (it is necessary to have an air compressor, and both fluids need to be heated in order to avoid recondensation phenomena); and PA1 installations are complex, and as a result very expensive, and they require specialist personnel for maintenance and adjustment. PA1 the device comprises: an enclosure defining first and second coaxial chambers; a first duct opening out into the first chamber to feed it with a first gas at medium pressure; a second duct opening out into the second chamber to feed it with a second gas at medium pressure; a first sonic nozzle having a throat of varying section disposed in the first chamber; and a second sonic nozzle having a throat of varying section disposed in the second chamber; PA1 the first sonic nozzle comprises a first hollow converging-diverging body of revolution serving as a seat for a first cone-shaped valve member, and the second sonic nozzle comprises a second hollow converging-diverging body of revolution serving as a seat for a second cone-shaped valve member; PA1 the first valve member has an axial bore and defines at least a portion of the first and second chambers; PA1 the second sonic nozzle is disposed inside said axial bore; PA1 the first and second hollow converging-diverging bodies of revolution are of determined position relative to the enclosure while the first and second valve members are mechanically linked to each other and are associated with a single actuator ensuring synchronized displacement of the first and second valve members; and PA1 the first and second gases after passing through the coaxial and geometrically similar first and second sonic nozzles flow axially into a common downstream chamber in which the first and second gases mix in a predetermined mixing ratio determined by the dimensions of the first and second sonic nozzles.
In the case of generators that generate a mixture of air and LPG, e.g. air/propane mixers, two different techniques are presently known. The so-called high or medium pressure technique uses compressed air and LPG at a pressure of a few bars, and it is capable of delivering powers that are quite high. The second or "low pressure" system uses atmospheric air and LPG at a pressure of a few bars.
Mixers for use with a distribution network operating at medium pressure (e.g. 2 bars) use compressed air and LPG at a pressure that generally lies in the range 4 bars to 10 bars. Such known mixers have a first line for feeding compressed air and a second line for feeding LPG.
The two lines come together in a manifold where mixing takes place prior to delivery at a pressure lying in the range a few millibars to 1 or 2 bars. The pressure limit is associated with the danger of LPG condensing when the weather conditions at the delivery site drop to low temperatures.
The mixing ratio is obtained by means of pneumatically-controlled proportional valves. Each line is fitted with a regulator valve controlled by a pneumatic device which gives rise to a simultaneous reaction of both valves, which valves are of sections and of opening relationships that are predefined. Recent systems make use of servo-control which, while maintaining the previous architecture, makes it possible significantly to improve the performance of a mixer of this type in terms of HCV accuracy and in terms of dynamic range. Such servo-control makes use of information concerning instantaneous flow rate as delivered by spinner meters disposed in each line, and information from a Wobbemeter which acts on the ratio setting. A controller associated with a computer serves to handle the various parameters.
Those various systems share the following defects:
Generators for generating mixtures of air and LPG for low pressure applications generally, make use of a battery of Venturi nozzles providing unit flow rates that can be combined to generate an arithmetic progression (e.g.: 10-20-40-80 m.sup.3 /h) so as to provide regulation in discrete steps. Mixing is obtained by sucking in atmospheric air, by the induction effect of the LPG jet which draws the air into the nozzle. The on/off mode of operation of the various nozzles makes it necessary to use buffer gasholders to "smooth" the resulting pressure in the network.
The advantage of that type of generator is that it does not require a supply of energy since it operates solely by using the pressure of the LPG.
The drawbacks are associated with the need to have a gasholder, which by its nature is bulky and expensive, in order to regulate pressure which would otherwise be unstable because of the cyclical operation of the nozzles. Finally, the precision obtained in terms of HCV is low since installations of that type generally have nozzles which are adjusted once and forever without making use of devices for implementing pressure and temperature corrections.